Orthogonal frequency division multiplexing (OFDM) is a method of multi-carrier digital communication where wideband information data is distributed across many narrow-band “subcarriers” in the frequency domain. Because each individual subcarrier is a narrowband sinusoidal wave, it can be transmitted with less distortion caused by inter-symbol-interference over multipath wireless fading environment. The original information can be recovered at the receiver without a complex equalization process, and is robust in a multipath propagation environment.
Orthogonal frequency division multiple access (OFDMA) is a method of multi-user digital communications, wherein each user utilizes an allotment of the available sub-carriers for their individual communication. The fundamental operation and advantages of OFDMA systems is similar to that of OFDM systems in multipath wireless propagation environment. So the terms OFDM and OFDMA are used here interchangeably.
An accurate channel estimation in an OFDM receiver is important for the recovery of the transmitted information data at the receiver. If the receiver makes a significant error in its channel estimation, the original modulation symbol can be decoded in error because each subcarrier in the OFDM symbol is multiplied by fading coefficients that have different amplitudes and phases. This is especially true for higher-order 16-QAM and 64-QAM modulation, which are widely used to transmit high data rate signals.
One method of estimating the channel response in the frequency domain is to measure the received pilot subcarriers. In each transmitted OFDM symbol, known pilots are inserted at known subcarrier locations. This helps later to estimate the channel values in those subcarrier locations. The channel values in other subcarrier locations can then be interpolated from the channel estimates for the received pilot subcarriers. Conventional interpolation methods include least square, linear, cubic and polynomial interpolation schemes.
Some communication systems do not evenly space the pilot subcarriers in every OFDM symbol, and the pilot subcarriers can be sparsely distributed across multiple OFDM symbols. However, these kinds of pilot patterns are usually repeated after several symbols.
One such example is a multi-input multi-output (MIMO) wireless communication system where multiple transmit and receive antennas are deployed between the communication links in order to increase the data rate and quality of service (QoS). In MIMO systems, the pilot subcarriers are often orthogonally partitioned between different transmit antennas to help the receiver track and estimate the different channel responses from each of the many transmit antennas, which results in sparsely spaced pilot subcarriers for every OFDM symbol. Another reason to use sparse pilot spacing across multiple symbols is to reduce the pilot overhead. Using fewer pilots reduces overhead, and so lesser transmit power can be used with less interference to other users.
Thus, there is a need for a device and method for minimizing channel estimation errors in OFDM-based wireless communication systems that use sparsely populated pilots.